This invention relates generally to defect detection systems, and, more particularly, to apparatus and a related method for detecting defects in manufactured parts, such as turbine blades, automobile engine parts, and other components that can fail under mechanical stress because of some inherent structural defect. Turbine blades in jet engines, for example, are subjected to substantial mechanical stresses under demanding conditions of temperature and pressure. Under these conditions, a defective part can fail without warning, often with potentially dangerous consequences. Defects in metal parts fabricated by casting and other processes are usually detectable as anomalies in the surface of the part. Defects caused by metal fatigue may also show up as surface anomalies. Surface anomalies may be cracks or pits, but they are often so small as to be invisible to the human eye under normal lighting conditions.
A number of processes have been developed to aid in the visual inspection of parts for surface defects. Some of these are known by the proprietary names Magnaflux, Magnaglo and Zyglo. Basically each of these processes requires that the part be first coated with a material that improves the contrast between the possible defects and the remaining or background area of the part. The part under inspection is illuminated, usually with ultraviolet light, which produces visible radiation when it impinges on the coating material, and an operator inspects it thoroughly for defects. Even with the use of these defect enhancement techniques such as these, there is still a significant change that defects will go undetected, mainly as a result of operator inattention and fatigue. To ensure practically complete freedom from defects, a batch of parts may have to be inspected several times over.
In some defect detection schemes of the prior art, a video camera is used to convert the defect images to analog electrical signals for display on a screen, and the images may then be enhanced by means of electronic image processing techniques. However, a decision as to whether the part should be rejected or not is still made as a result of inspection of the enhanced image by an operator.
It will be apparent from the foregoing that there has been a significant need for an improved defect detection system which relieves the operator of the burden of deciding whether a surface anomaly constitutes a rejectable defect. The present invention fulfills this need.